After birth, the foramen ovale closes to form which structure?

• A. Fossa ovalis
• B. Ligamentum arteriosum
• C. Crista terminalis
• D. Interventricular septum

Answer: (A)

After birth, the foramen ovale closes as the lungs expand and left atrial pressure rises, pushing the septum primum against the septum secundum to stop the right-to-left shunt. This functional closure becomes permanent as the two septa fuse, leaving a fibrous remnant that appears as a shallow depression in the interatrial septum called the fossa ovalis. The rim around this depression is the limbus fossa ovalis. This remnant marks where the opening used to be. The ligamentum arteriosum is the remnant of the ductus arteriosus, the crista terminalis is a muscular ridge in the right atrium, and the interventricular septum is the wall between the ventricles. So the structure formed is the fossa ovalis.